In the manufacture of integrated circuits a monolithic structure is first built typically on a single crystalline silicon substrate by successive steps including material deposition, etching, and lithographic definition. That is, successive layers of materials as, for example, insulators, semiconductor materials, and metals are patterned to produce desired circuitry. At the top of this successive stacking of layers there is typically a multiplicity of bonding pads used for connecting the integrated circuit to the environment of a larger electronic assemblage. For example, as shown in FIG. 1, in plan view, the upper surface of the integrated circuit structure, 10, is shown with bonding pads, 11.
This integrated circuit structure, typically denominated a die, presently has from 10 to 10000 bonding pads with the space between adjacent bonding pads between 1 and 1000 μm. This shortest distance from the center of one bonding pad to the center of an adjacent bonding pad is defined as the pitch of the bonding pad array. (Center in this context is defined as the point C of Cartesian coordinates X and Y that satisfy the two relationships∫Ax∂A=Ax and ∫Ay∂A=Aywhere A is the area of the pad. Such integral relationships are particularly simple for symmetric pads. For example, for a square, C is at x=½ length and y=½ width; for a circle C is at x=½ diameter and y=½ diameter.) Although such bonding pads are typically ordered in perpendicular rows and columns, such regular ordering is not critical and the term array subsumes all bond pad configurations.
The die is packaged in a structure, 20, shown in FIG. 2. The die, 10, with bonding pads, 11, is adhered to substrate, 15, using, for example, a polymer, 16, (e.g. epoxy polymeric die attach material that is sold by any of a number of commercial suppliers such as Ablestik Corporation located at Susana Road, Rancho Dominguez, Calif.). The bonding pads on the die are used to connect the interconnect die circuit metallization to external pads in the package, 12, with wires, 14. The wires are generally gold or high gold content alloys (typically at least 99% by weight gold) having diameters in the range 10 to 30 μm. The entire structure is then encapsulated in a material such as a polymer material, e.g. mold compound material that is sold by any of a number of commercial suppliers such as Nitto Denko Corporation of Herbis Osaka, 2-5-25, Umeda, Kita-Ku, Osaka 530-0001. This encapsulation is accomplished by molding processes such as injection molding involving a flow of polymeric material into a mold cavity containing the substrate with its attached die and attendant wire bonds. The polymer flow to fill the mold cavity exerts a meaningful force on bonding wires, 14. Thus these wires should have sufficient stiffness, as primarily determined by the wire diameter, to resist their deflection into neighboring wires attached to adjoining bonding pads. Such deflection (often denominated sweep) substantially increases the potential for short circuits between these adjacent wires.
Additionally the bonding of wire, 14, to pad, 11, is accomplished typically by thermosonic bonding. As shown in FIG. 3 substrate, 31, of the die, 10, has an upper layer, 32, generally formed from undoped silica glass by processes such as chemical vapor deposition or plasma assisted chemical vapor deposition and patterned using conventional lithographic processes. A copper region, 33, is formed by electroplating and an overlying passivating region, 34, is formed from materials such as silicon dioxide, silicon nitride, carbon doped silicon dioxide or nitride, or a combination of some or all the four again using conventional deposition and lithographic processes. The bonding pad, 11, is generally a patterned aluminum containing composition that is passivated by a patterned overlying layer, 36. (Passivating materials include, for example, silicon oxides, silicon nitride, and combinations of these two.) A capillary tube, 38, is used to bond the wire, e.g. gold or gold alloy wire, 39, to bonding pad, 11. A wire portion extending beyond tip, 30, of the capillary tube is formed into a ball, 37, by applying an electric arc as described in Wire Bonding in Microelectronics Materials Processes, Reliability, and Yield, 2nd Ed., G. Harman (1997). The capillary tube, 38, is then used to apply pressure to the ball, 37, onto bonding pad, 11 while introducing ultrasonic energy along the wire through the capillary tube. As a result, formation of Al/Au intermetallic composition is induced that provides a bond interface. Typically forces in the range of 2 to 30 grams are applied to balls formed from wires having respectively diameters in the range 10 to 30 μm. (The ball diameter is typically 1.5 to 4 times the diameter of the wire. In this context, irrespective of the ball shape, its diameter is considered that of a sphere having a volume equal to that of the ball.)
As previously discussed, to avoid short circuits due to wire sweep it is desirable to employ larger wire diameters. However, as packing densities increase, bonding pad size and pitch decrease. Therefore, the physical dimensions of the die and its bonding pad array limit the diameter of the wire employed in bonding. As an accommodation to these competing considerations, generally wire diameters in the range 10 μm to 30 μm resulting in balls having diameters in the range 15 to 120 μm are employed with bonding pads having lateral dimension, 40, between edges, 41 of the passivation region, 36 overlying bonding pad, 11. Accordingly, in general a significant portion of aluminum bonding pad extends beyond the region encompassed by the diameter of ball, 37 but the wire diameter is sufficient to acceptably limit the probability of short circuit formation.
Despite the accommodations made, other problems still arise. Corrosion at the wire ball/bonding pad interface has been observed especially when the wire bonded region of the pad is exposed to corrosive materials such as bromine that are added to the mold compound to improve flame resistance. To avoid such corrosion, there has been a trend away from bromine entity containing materials. Unfortunately other corrosive elements such as moisture are omnipresent. In particular, commonly used molding compounds are fully permeated within a few days to a month under typical environmental temperature and humidity conditions. Other corrosive elements such as Cl and/or Na often remain in the molding compound after processing. Such Na and/or Cl in combination with moisture readily corrode Al and Al/Au compounds. Thus, it is a challenge to produce an adequate device even with substantial material and configuration compromises.